3 Answers
3

Yeast produce different flavors during the various stages of their lifecycle. Underpitching lengthens their "growth" phase (maybe a better name is "division" or "budding"). The bulk of a beer's esters are produced during this initial stage, so extending this part of the lifecycle increases this sometimes undesirable quality.

Yeast need oxygen to bud. A homebrewer can usually dissolve enough oxygen into wort to facilitate four buddings. Once out of oxygen yeast begin consuming sugars. In this phase the yeast stores energy to sustain it during hibernation. Yeast will begin consuming the simple sugars moving to the more complex molecules as the easy to digest food diminishes. Once it is full of stored energy the cell shuts down and flocculates.

Underpitching can introduce an insufficient amount of yeast to entirely consume the entirety of the beer's fermentables. Your fermentation may not finish, or there could be enough food for an infection to take hold.

I found out that I've been drastically underpitching my first few batches. I'm fairly ready to blame my under-attenuated beers on this. Here's why:

I started with a dry yeast pack
(Danstar Nottingham Ale) back when I
was just a kid and didn't know
anything... read: December. I pitched
direct into the wort and got 70%
apparent attenuation. That batch had
one problem though: the dreaded
banana flavor, which didn't
actually emerge until after bottle
conditioning.

That led me to believe that it was
the yeast that
was the culprit for the banana
flavor. I switched to Wyeast smack
packs, all purchased from the LHBS,
and believed the printing on the pack
that it was good for pitching one
five-gallon batch up to 1.050 OG. I think my pitch rates went down significantly because of this.

This led me to a series of
under-attenuated beers. Strangely,
I've had one beer that went over the
attenuation range for its yeast (a
maibock fermented with Cry
Havoc). The fact that this came
from a different supplier (White
Labs) and was maybe fresher, made me
think that under-pitching was a
problem. Accordingly, I've edited my
answer to the yeast re-use
question to recommend more slurry and shorter storage.

For my next seven batches, I'm moving to a series-based approach, probably using a White Labs American Ale Blend to get a solid yeast cake on a cream ale, then re-using most of that slurry in three generations of two batches each, with the last two batch generation being two high-gravity beers pitched directly onto the yeast cake of the previous batch. (Is "pitched" the right word here? Spawning new question.)

More ester formation. This could be good in a Belgian beer where you want to accentuate the yeast-derived flavors, or bad such as in the case of lagers. Generally it's best to pitch the right amount of yeast and control ester production with fermentation temperature.